[#493] node/config: Fix corrupting of path to the subsection

In previous implementation `Config.Sub` method could lead to the violation
of the internal `path` slice because of `append`. This has been observed on
deeply nested subsections.

Fix `Config.Sub` to copy internal slice in order to prevent violations.
Cover problem case in test config files and unit test.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

cmd/neofs-node/config/calls.go

@@ -8,9 +8,16 @@ import (
 //
 // Returns nil if subsection if missing.
 func (x *Config) Sub(name string) *Config {
+	// copy path in order to prevent consequent violations
+	ln := len(x.path)
+
+	path := make([]string, ln, ln+1)
+
+	copy(path, x.path)
+
 	return &Config{
 		v:    x.v,
-		path: append(x.path, name),
+		path: append(path, name),
 	}
 }
 

cmd/neofs-node/config/calls_test.go

@@ -42,3 +42,21 @@ func TestConfigEnv(t *testing.T) {
 
 	require.Equal(t, value, c.Sub(section).Value(name))
 }
+
+func TestConfig_SubValue(t *testing.T) {
+	configtest.ForEachFileType("test/config", func(c *config.Config) {
+		c = c.
+			Sub("section").
+			Sub("sub").
+			Sub("sub")
+
+		// get subsection 1
+		sub := c.Sub("sub1")
+
+		// get subsection 2
+		c.Sub("sub2")
+
+		// sub should not be corrupted
+		require.Equal(t, "val1", sub.Value("key"))
+	})
+}

cmd/neofs-node/config/test/config.json

@@ -1,7 +1,18 @@
 {
   "value": "some value",
+
   "section": {
-    "any": "thing"
+    "any": "thing",
+    "sub": {
+      "sub": {
+        "sub1": {
+          "key": "val1"
+        },
+        "sub2": {
+          "key": "val2"
+        }
+      }
+    }
   },
 
   "string_slice": {

cmd/neofs-node/config/test/config.yaml

@@ -3,6 +3,14 @@ value: some value
 section:
   any: thing
 
+  sub:
+    sub:
+      sub1:
+        key: val1
+
+      sub2:
+        key: val2
+
 string_slice:
   empty: []